Apparatus and method of treating exhaust gas

ABSTRACT

An exhaust gas treating apparatus  1  includes; a case body  2  and a plasma producing means  3  capable of producing plasma inside the case body  2  and treats the substances to be treated contained in the exhaust gas by the plasma producing means  3 . The plasma producing means  3  has one or more each of a pulse electrode  4  and a ground electrode  5  that are oppositely disposed in the case body  2  and has a pulse power source  6  capable of feeding a pulse current to the pulse electrode  4  by switching frequency and/or voltage for different values at predetermined time intervals. The substances to be treated contained in the exhaust gas can selectively be treated by switching frequency and/or voltage value for different values at predetermined time intervals so that plasma of a kind adequate for the substances to be treated contained in an exhaust gas is produced between the pulse electrode  4  and the ground electrode  5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit of PCT Application No.PCT/JP2004/009016, entitled “Apparatus And Method Of Treating ExhaustGas,” by Yukio Miyairi, Yasumasa Fujioka, Masaaki Masuda, TatsuhikoHatano, Takeshi Sakuma, Yuuichiro Imanishi, Keizo Iwama, and KenjiDosaka, filed on Jun. 25, 2004, which is based on Japanese ApplicationNo. 2003-184286, entitled “Apparatus And Method Of Treating ExhaustGas,” by the same inventors, filed on Jun. 27, 2003. All of the aboveapplications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to an exhaust gas treating apparatus andan exhaust gas treating method. More particularly, the present inventionrelates to an exhaust gas treating apparatus and an exhaust gas treatingmethod, which are capable of selectively treating substances to betreated contained in an exhaust gas.

BACKGROUND OF THE INVENTION

There have been known that silent discharge is caused by applying ahigh-voltage alternating current or a periodic pulse voltage on adielectric body disposed between two electrodes to generate a plasmafield, where an active species, a radical, and an ion are formed topromote reaction and decomposition of gas and that this can be used forremoving harmful components contained in engine exhaust gas or exhaustgas from various kinds of incinerators.

For example, there is disclosed a plasma exhaust gas treating systemwhich treats, e.g., NO_(x), carbon particulate, HC, and CO contained inengine exhaust gas or exhaust gas from various kinds of incinerators bypassing the engine exhaust gas or exhaust gas from various kinds ofincinerators through the plasma field (see, e.g., Japanese PatentLaid-open No. 164925/2001).

DISCLOSURE OF THE INVENTION

However, there arose a problem that each of a plurality of substances tobe treated contained in exhaust gas requires independent electrode forgenerating plasma adequate for each substance when an exhaust gascontaining a plurality of substances because intensity of plasmaadequate for treating substances differs depending on each of thesubstances to be treated contained in exhaust gas, and thereby plasmaaiming to treat a predetermined substance cannot treat the othersubstances. In addition, there arose another problem of increasingelectricity consumption because highly intensive plasma has to begenerated in the case that a plurality of substances is treated with onekind of plasma using a pair of electrodes. Further, there arose anotherproblem that a NO_(x) reduction catalyst, which is used together withplasma and which needs fuel (hydrocarbon) contained in exhaust gas forcatalyst reaction, cannot sufficiently exhibit its ability becausehydrocarbon is completely oxidized by plasma when the NO_(x) reductioncatalyst is used together with highly intensive plasma.

The present invention has been made in view of the above problems andprovides an apparatus and a method of treating exhaust gas capable ofselectively treating substances to be treated contained in the exhaustgas.

In order to achieve the above aim, according to the present invention,there is provided the following apparatus and method of treating exhaustgas.

[1] An exhaust gas treating apparatus comprising:

a case body functioning as a passage of exhaust gas containingsubstances to be treated, and

a plasma producing means capable of producing plasma inside the casebody; and treating the substances to be treated contained in the exhaustgas by the plasma produced by the plasma producing means;

wherein the plasma producing means has one or more each of a pulseelectrode and a ground electrode that are oppositely disposed in thecase body and has a pulse power source capable of feeding a pulsecurrent to the pulse electrode, and by switching frequency and/orvoltage value for different values at predetermined time intervals sothat plasma of a kind adequate for the substances to be treatedcontained in an exhaust gas is produced between the pulse electrode andthe ground electrode, the substances to be treated in the exhaust gascan selectively be treated (hereinafter sometimes referred to as “firstinvention”).

[2] An exhaust gas treating apparatus according to the above [1],wherein the predetermined time intervals are 0.01 to 500 seconds.

[3] An exhaust gas treating apparatus according to the above [1] or [2],wherein the pulse power source can feed a pulse current by switchingfrom a first pulse current having a frequency of 100 to 1000 Hz to asecond pulse current having a frequency of 500 to 2500 Hz, and viceversa.

[4] An exhaust gas treating apparatus according to any one of the above[1] to [3], wherein the pulse power source can feed a pulse current byswitching from a third pulse current having a voltage value of 2 to 5 kVto a fourth pulse current having a voltage value of 3 to 20 kV, and viceversa.

[5] An exhaust gas treating apparatus according to any one of the above[1] to [4], wherein the pulse electrode and/or the ground electrodecomprise(s) a ceramic body functioning as a dielectric body and aconductive film disposed in the ceramic body.

[6] An exhaust gas treating apparatus according to any one of the above[1] to [5], which further comprises a catalyst on a downstream side ofthe plasma producing means in the passage of exhaust gas.

[7] An exhaust gas treating method treating substances to be treatedcontained in exhaust gas by plasma by producing the plasma in a passageof the exhaust gas containing the substances to be treated, wherein oneor more each of a pulse electrode and a ground electrode are oppositelydisposed, and a pulse current is fed by switching frequency and/orvoltage value for different values at predetermined time intervals sothat plasma of a kind adequate for the substances to be treatedcontained in the exhaust gas is produced between the pulse electrode andthe ground electrode to selectively treat the substances to be treatedin the exhaust gas (hereinafter sometimes referred to as “secondinvention”).

[8] An exhaust gas treating method according to the above [7], whereinthe predetermined time intervals are 0.01 to 500 seconds.

[9] An exhaust gas treating method according to the above [7] or [8],wherein the pulse power source can feed a pulse current by switchingfrom a first pulse current having a frequency of 100 to 1000 Hz to asecond pulse current having a frequency of 500 to 2500 Hz, and viceversa.

[10] An exhaust gas treating method according to any one of the above[7] to [9], wherein the pulse power source can feed a pulse current byswitching from a third pulse current having a voltage value of 2 to 5 kVto a fourth pulse current having a voltage value of 3 to 20 kV, and viceversa.

[11] An exhaust gas treating method according to any one of the above[7] to [10], wherein the exhaust gas is exhausted from an automobileengine, and the pulse current is supplied by switching, step-by-step,frequency and/or voltage for different values at predetermined timeintervals in correspondence with change in revolution and/or load of theengine.

[12] An exhaust gas treating method according to any one of the above[7] to [11], wherein the exhaust gas is exhausted from an automobileengine, and the predetermined time intervals for switching frequencyand/or voltage value for different values is changed step-by-step incorrespondence with change in revolution and/or load of the engine.

[13] An exhaust gas treating method according to any one of the above[7] to [12], wherein a catalyst is further disposed on a downstream sideof the portion where the plasma is produced in the passage of exhaustgas to further treat exhaust gas passed through the plasma by thecatalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an embodiment of an exhaust gastreating apparatus of the present invention (first invention).

FIG. 2 is a schematic view showing another embodiment of an exhaust gastreating apparatus of the present invention (first invention).

FIG. 3( a) is a schematic view showing a process of collecting soot onthe ground electrode in an embodiment of an exhaust gas treatingapparatus of the present invention (first invention).

FIG. 3( b) is a schematic view showing a process of collecting soot onthe ground electrode in an embodiment of an exhaust gas treatingapparatus of the present invention (first invention).

BEST MODE FOR CARRYING OUT THE INVENTION Examples

Embodiments of an apparatus and a method of treating an exhaust gas ofthe present invention will hereinbelow be described in detail based onFigures. However, the present invention is not limited to theembodiments, and various changes, modifications, and improvements may begiven based on the knowledge of those in the art as long as they deviatefrom the scope of the present invention.

In the first place, an exhaust gas apparatus of an embodiment of thepresent invention (first invention) is described. As shown in FIG. 1, anexhaust gas treating apparatus 1 of the present embodiment is providedwith a case body 2 functioning as a passage of exhaust gas containingsubstances to be treated and a plasma producing means 3 capable ofproducing plasma inside the case body 2, and treats the substances to betreated contained in the exhaust gas by the plasma produced by theplasma producing means 3; wherein the plasma producing means 3 has oneor more each of a pulse electrode 4 and a ground electrode 5 that areoppositely disposed in the case body 2 and has a pulse power source 6capable of feeding a pulse current to the pulse electrode 4, and byswitching frequency and/or voltage value of the pulse current fed fromthe pulse power source 6 for different values at predetermined timeintervals so that plasma of a kind adequate for the substances to betreated contained in an exhaust gas is produced between the pulseelectrode 4 and the ground electrode 5, the substances to be treated inthe exhaust gas can selectively be treated.

In addition, an exhaust gas treating apparatus of the present embodimentmay have the constitution further provided with a catalyst 10 on thedownstream side of the plasma producing means 3 in an exhaust gaspassage, for example, as shown in FIG. 2. By employing a catalyst whichpromotes NO_(x) decomposition reaction as the catalyst 10, it becomespossible to treat NO₂, or the like, which is difficult to be treated byplasma, by decomposition (reduction), and thereby exhausted gas can bedischarged outside in cleaner condition.

For example, in the present embodiment, it is preferable that the pulsepower source 6 can feed a current by switching at predetermined timeintervals the first pulse current (hereinafter sometimes referred to asa “low-frequency pulse current”) having a relatively low frequency andcapable of producing plasma adequate for treating nitrogen monoxide(NO), which is one of the substances to be treated contained in anexhaust gas, and the second pulse current (hereinafter sometimesreferred to as a “high-frequency pulse current”) having a relativelyhigh frequency and capable of producing plasma adequate for treatingsoot, which is one of the substances to be treated contained in exhaustgas. By this constitution, it is possible to oxidize nitrogen monoxide(NO) to nitrogen dioxide (NO₂) by plasma produced by a low-frequencypulse current (hereinafter sometimes referred to as “low-frequencyplasma) and to produce well aldehyde, or the like, effective inimproving performance of a NO_(x) reduction catalyst by moderatelyoxidizing hydrocarbon (HC) contained in an exhaust gas. In addition,though it is almost impossible that low-frequency plasma treats sootcontained in exhaust gas, it is possible to inhibit soot from beingdischarged outside the case body 2 without being treated because sootcan be collected on the ground electrode 5 side by electrostaticcollecting.

The above electrostatic collecting is conducted in such a manner that anelectron 12 adheres to soot 11, the soot 11 is negatively electrified,and the soot 11 to which the electron 12 adheres is drawn on thepositive electrode side, i.e., the ground electrode 5 side as shown inFIG. 3( a).

Plasma produced by a high-frequency pulse current (hereinafter sometimesreferred to as “high-frequency plasma”) switched from a low-frequencypulse current can treat by oxidation at a stroke to the soot 11collected on the ground electrode 5 side and the soot 11 contained inthe exhaust gas newly flew in with high-frequency plasma having highoxidizability as shown FIG. 3( b). Needless to say, nitrogen monoxide(NO) can be treated by oxidation to give nitrogen dioxide (NO₂) with thehigh-frequency plasma. However, since the high-frequency plasma has highoxidizability, most of hydrocarbon (HC) is completely oxidized by carbondioxide (CO₂) and water.

In a conventional exhaust gas treating apparatus, a kind of plasmatreatment was conducted in the case of treating substance to be treatedusing plasma. However, since an oxidizing treatment on soot requiringrelatively high oxidizability and effective production of aldehyde, orthe like having an effect in improving performance of a NO_(x) reductioncatalyst are antinomic as described above, they could not standtogether.

In an exhaust gas treating apparatus of the present embodiment, sinceplasma having different conditions is produced at predetermined timeintervals, substances to be treated can selectively be treated with atleast one pair of electrodes (a pulse electrode 4 and a ground electrode5) by producing aldehyde or the like having an effect in improvingperformance of a NO_(x) reduction catalyst with treating NO on the onehand and treating soot requiring oxidizability on the other hand.

In an exhaust gas treating apparatus 1 of the present embodiment asshown in FIG. 1, excited oxygen, which is excited by plasma actuallyoxidizes the substances to be treated contained in the exhaust gaspassing between the pulse electrode 4 and the ground electrode 5, andthe amount of the oxygen in the excited state closely relates tooxidizability of the plasma. Oxygen in the excited state includestriplet excited oxygen (3P), which is stable and has low oxidizability,and singlet excited oxygen (1D), which is unstable and highoxidizability. More singlet excited oxygen (1D) is present inhigh-frequency plasma than in low-frequency plasma. Therefore,low-frequency plasma is adequate for oxidation of NO, and high-frequencyplasma is adequate for oxidation of soot.

However, since the singlet excited oxygen (1D) is unstable and has ashort life span, sometimes the singlet excited oxygen (1D) is notdiffused widely between the electrodes to exhibit insufficientoxidizability. In particular, oxidizability is sometimes further loweredbecause an electron sheath where plasma is hardly produced is formed inthe periphery of soot collected by the electrode.

Therefore, it is preferable that an exhaust gas treating apparatus 1 ofthe present embodiment not only directly produce the singlet excitedoxygen (1D) with high-frequency plasma, but also produce ozone (O₃) withplasma produced between the electrodes and photodissociate the obtainedozone by emission of plasma to produce more singlet excited oxygen (1D).Specifically, ozone (O₃) is dissociated to an oxygen molecule (O₂) andsinglet excited oxygen (1D) with light having a wavelength of 310 nm orless, and therefore, it is possible to produce more singlet excitedoxygen (1D) by dissociating ozone (O₃) produced by low-frequency plasmawith emission of high-frequency plasma by making the exhaust gastreating apparatus 1 possible to feed a pulse current by switching thepulse power source 6 between a pulse current having a frequency capableof obtaining low-frequency plasma adequate for effectively producingozone (O₃) and a pulse current having a frequency capable of obtaininghigh-frequency plasma adequate for emission of light containing muchlight having a wavelength of 310 nm or less. Further, by using ozone(O₃), the singlet excited oxygen (1D) is effectively produced even inthe electron sheath where plasma is hardly produced.

It is preferable that the pulse power source 6 used in the presentembodiment can feed a pulse current by switching the first pulse currenthaving a frequency of 100 to 1000 Hz and the second pulse current havinga frequency of 500 to 2500 Hz for effectively obtaining theaforementioned function and effect.

The pulse power source 6 used in the present embodiment may be able tofeed a pulse current by switching to different voltage value atpredetermined time intervals with keeping frequency the same.Alternatively, it may be able to feed a pulse current by switching todifferent frequency and different voltage value at predetermined timeintervals. In the case that a pulse current is fed by switching todifferent voltage value, the apparatus should be able to switch thepulse power source 6 between the third pulse current (hereinaftersometimes referred to as a “low-voltage pulse current”) havingrelatively low voltage value and being capable of producing plasmaadequate for treating nitrogen monoxide (NO), which is one of thesubstances to be treated contained in an exhaust gas and the fourthpulse current (hereinafter sometimes referred to as a “high-voltagepulse current”) having relatively high voltage and being capable ofproducing plasma adequate for treating soot, which is one of thesubstances to be treated contained in an exhaust gas. By thisconstitution, the same function and effect as in the aforementioned casehaving different frequencies can be obtained. In particular, by changinga low-frequency pulse current to low voltage to generate more ozone withlow-frequency low-voltage plasma and further changing a high-frequencypulse current to high-voltage to increase emission of high-frequencyhigh-voltage plasma and promote dissociation of ozone, efficiency ofoxidizing soot contained in an exhaust gas can further be improved.

In the case when pulse currents having different voltage values are fed,the voltage value of each pulse current is not particularly limited. Forexample, in the case of treating an automobile exhaust gas, the pulsepower source 6 is preferably able to feed a pulse current by switchingthe third pulse current of 2 to 5 kV (low-voltage pulse current) and thefourth pulse current of 3 to 20 kV (high-voltage pulse current). Whenthe voltage value of the third pulse current (low-voltage pulse current)is below 2 kV, oxidation of NO is sometimes insufficient. When thevoltage value of the third pulse current (low-voltage pulse current) isabove 5 kV, hydrocarbon is sometimes oxidized completely. When thevoltage value of the fourth pulse current (high-voltage pulse current)is below 3 kV, oxidation of soot is sometimes insufficient. When thevoltage value of the fourth pulse current (high-voltage pulse current)is above 20 kV, the pulse power source 6 is too large, which causes aproblem of mounting and sometimes causes a problem of electromagnetictrouble to the circumference.

Though a time interval for switching frequencies and/or voltage valuesdepends on the size of the exhaust gas treating apparatus 1, eachfrequency, each voltage value, etc., it is preferably 0.01 to 500seconds, and more preferably 0.01 to 10 seconds. A time interval forswitching the above-mentioned frequencies may be determined by a pulsenumber in each condition. For example, in the case of treating anautomobile exhaust gas with feeding a pulse current by switching betweentwo kinds of frequencies, the time interval is predetermined in such amanner that the pulse number of the low-frequency pulse current ispreferably 1 to 50000 times, more preferably 3 to 50000 times, withrespect to one pulse of the high-frequency pulse current having higherfrequency. When the pulse number of the low-frequency pulse current withrespect to one pulse of the high-frequency pulse current is below 1,production of aldehyde, or the like effective in improving performanceof a NO_(x) reduction catalyst is sometimes insufficient. When the pulsenumber of the low-frequency pulse current with respect to one pulse ofthe high-frequency pulse current is above 50000, time for collectingsoot on the ground electrode 5 becomes too long, and the soot sometimescannot be treated completely by high frequency plasma after switchingfrequency. Incidentally, the same applies in the case of voltage value.

As length (period) of a pulse of a pulse current fed from the pulsepower source 6 used in the present embodiment, in the case of feeding apulse current with switching two kinds of frequency, the timecorresponding with a half width of the maximum voltage value of onepulse is preferably 0.8 to 200 μsec., more preferably 0.8 to 50 μsec.for the length (period) of one pulse having high frequency. When thelength of one pulse of a pulse current is below 0.8 μsec., efficiency ofoxidizing soot is sometimes lowered. When it is above 50 μsec., aconsumption amount of electric power sometimes increases. For the length(period) of one pulse having low frequency, the time corresponding witha half width of the maximum voltage value of one pulse is preferably 1to 500 μsec., more preferably 1 to 100 μsec. When the length of onepulse of a pulse current is below 1 μsec., efficiency of oxidizing NO issometimes lowered, and an effect of electrostatic collection issometimes insufficient. When it is above 50 μsec., efficiency ofelectric power is sometimes lowered.

A pulse power source 6 used in the present embodiment is notparticularly limited as long as it can feed a pulse current as describedabove. For example, there may suitably be used a pulse power sourceusing a thyristor, a pulse power source using a transistor other than athyristor, or the like.

Though there is no particular limitation on constitution of the pulseelectrode 4 and the ground electrode 5 used in the present embodiment,it is preferable that the constitution can produce plasma uniformlybetween the electrodes. For example, as the electrode on the negativeelectrode (pulse electrode 4) side, there may suitably be used anelectrode constituted by a ceramic body functioning as a dielectric bodyand a conductive film disposed inside the ceramic body. Though theelectrode on the positive electrode (ground electrode 5) side ispreferably a conductive body from the viewpoint of maintainingelectrostatic collection, it may be an electrode having the samestructure as the negative electrode. In addition, though distancebetween the pulse electrode 4 and the ground electrode 5 is notparticularly limited, it is preferably 0.3 to 5 mm so that plasma caneffectively be produced between oppositely disposed electrodes. When thedistance is above 5 mm, effect in collecting dust is sometimesinsufficiently obtained. In addition, it is preferable that a pluralityof pulse electrodes 4 and ground electrodes 5 are disposed, for example,in a laminated condition alternately so that a ground electrode 5 facesa pulse electrode 4 on each side of the pulse electrode as shown inFIG. 1. The number of the pulse electrodes 4 and the ground electrodes 5is not particularly limited.

Though there is no particular limitation on material for a case body 2used in the present embodiment, in the case of disposing an exhaust gastreating apparatus 1 of the present embodiment in, for example, anautomobile, a conductive metal is used as the material for the case body2. It is preferable to connect a part of the case body 2 with the groundelectrode 5 and further with a ground line of the automobile, or thelike electrically.

In the exhaust gas treating apparatus 1 shown in FIG. 2, a plasmaproducing means 3 disposed on the exhaust gas generating side (upstreamside) of an exhaust gas passage is connected with a catalyst 10 disposedon the exhaust side via a pipe 14. Incidentally, there may be employedthe constitution where the catalyst 10 on the downstream side isdirectly connected with the plasma producing means 3 in the presentembodiment.

The exhaust gas treating apparatus 1 shown in FIG. 2 can purify, forexample, NO_(x) in an exhaust gas in an oxygen excess atmosphere. Thatis, with plasma produced by the plasma producing means 3, NO_(x) isreformed so that it is easily purified by the catalyst 10 on thedownstream side, or HC (hydrocarbon), or the like in an exhaust gas isreformed so that it easily reacts with NO_(x), and NO_(x) is purified bythe catalyst 10.

The plasma producing means 3 used in the exhaust gas treating apparatus1 converts NO_(x) in exhaust gas generated by combustion of a lean burn,gasoline-injection engine, a diesel engine, or the like, in an oxygenexcess atmosphere into NO₂ by plasma. In addition, the plasma producingmeans 3 produces an active species from HC, or the like in an exhaustgas, and constituted in a manner similar to the plasma producing means 3shown in FIG. 1 can suitably be employed.

The catalyst 10 is disposed on the downstream side of the plasmaproducing means 3 in the exhaust system as a catalyst unit 13 providedwith a catalyst member including a substrate having a plurality of finepores where an exhaust gas passes. The catalyst member has a substrateand a catalyst layer formed so as to cover the inner wall surfacesurrounding the plurality of fine pores of the substrate. The catalystunit 13 is exemplified by a conventionally known NO_(x) treatingapparatus.

Since the catalyst layer is produced generally by immersing thesubstrate in a slurried catalyst (catalyst slurry) as described below,it is sometimes called a wash coat (layer).

A shape of the substrate is not particularly limited in the presentinvention as long as it has a space where an exhaust gas circulates, anda honeycomb-shaped substrate having a plurality of fine pores is used inthe present embodiment.

The substrate is preferably formed with material having thermalresistance. Examples of such material include ceramics such ascordierite, mullite, silicon carbide (SiC), and silicon nitride (Si₃N₄)and metals (e.g., stainless steel).

The catalyst layer is formed using a carrier of porous material and oneselected from Pt, Pd, Rh, Au, Ag, Cu, Fe, Ni, Ir, Ga, and the like, or acombination thereof which is loaded on a surface of the carrier ofporous material as the main portion. Inside the catalyst layer, aplurality of continuous fine pores which continue to the fine pores ofthe substrate.

The porous carrier can be produced by using one suitably selected from,for example, alumina, zeolite, silica, titania, zirconia, silicaalumina, ceria, and the like. Incidentally, a catalyst, which promotesdecomposition reaction of NO_(x) is used as the catalyst 10.

Next, an embodiment of an exhaust gas treating method of the presentinvention (second invention) is specifically described. An exhaust gastreating method of the present embodiment treats substances to betreated contained in an exhaust gas by plasma produced in a passage ofthe exhaust gas containing the substances to be treated, wherein one ormore each of a pulse electrode and a ground electrode are oppositelydisposed, and a pulse current is fed by switching frequency and/orvoltage value for different values at predetermined time intervals sothat plasma of a kind adequate for the substances to be treatedcontained in an exhaust gas is produced between the pulse electrode andthe ground electrode to selectively treat the substances to be treatedin the exhaust gas.

By this constitution, substances to be treated contained in an exhaustgas can selectively be treated by feeding a pulse current by switchingbetween a pulse current having low frequency (hereinafter sometimesreferred to as a “low-frequency pulse current”) adequate for treatingnitrogen monoxide (NO) contained in an exhaust gas and a pulse currenthaving high frequency (hereinafter sometimes referred to as a“high-frequency pulse current”) adequate for treating soot in the caseof, for example, treating an exhaust gas discharged from an automobileengine.

Plasma produced by a low-frequency pulse current (hereinafter sometimesreferred to as “low-frequency plasma”) can effectively treat nitrogenmonoxide (NO), which is a substance contained in exhaust gas byoxidation to give nitrogen dioxide (NO₂) and moderately oxidizehydrocarbon (HC) contained in an exhaust gas to produce aldehyde, or thelike, which has an effect in improving performance of a NO_(x) reductioncatalyst. In addition, though low-frequency plasma cannot treat sootcontained in an exhaust gas, since it can collect soot on the groundelectrode side by an effect of electrostatic collection, it caneffectively inhibit soot from being discharged outside without beingtreated.

Plasma produced by a high-frequency pulse current (hereinafter sometimesreferred to as “high-frequency plasma”) can give an oxidizing treatmentat a stroke to the soot collected on the ground electrode side and thesoot contained in the exhaust gas newly flew in with plasma having highoxidizability. Needless to say, nitrogen monoxide (NO) can be treated tobe oxidized to give nitrogen dioxide (NO₂) by the plasma having highoxidizability.

Therefore, according to an exhaust gas treating method of the presentembodiment, soot contained in an exhaust gas can effectively beoxidized, and aldehyde, or the like, having an effect in improvingperformance of a NO_(x) reduction catalyst can be produced, and therebyan exhaust gas can effectively be treated by an exhaust gas treatingmethod of the present embodiment, for example, as an exhaust gastreatment before the exhaust gas is introduced into a NO_(x) treatingapparatus using a NO_(x) reduction catalyst.

In an exhaust gas treating method of the present embodiment, a pulsecurrent is preferably fed by switching between the first pulse currenthaving a relatively low frequency of 100 to 1000 Hz and the second pulsecurrent having a relatively high frequency of 500 to 2500 Hz.

In the present embodiment, a pulse current may be fed by switchingvoltage value for different values at predetermined time intervals withkeeping frequency the same. Alternatively, a pulse current may be fed byswitching frequency and voltage value for different values atpredetermined time intervals. In the case of feeding a pulse current byswitching voltage value for different values, it is preferable toalternately feed the third pulse current having relatively low voltagevalue (hereinafter sometimes referred to as a “low-voltage pulsecurrent”) capable of producing plasma adequate for treating nitrogenmonoxide (NO) which is one of the substances to be treated contained inexhaust gas and the fourth pulse current having relatively high voltagevalue (hereinafter sometimes referred to as a “high-voltage pulsecurrent”) capable of treating soot which is one of the substances to betreated contained in exhaust gas. This constitution can give the samefunction and effect as the case having different frequencies describedabove. In particular, by changing a low-frequency pulse current to lowvoltage to generate more ozone with low-frequency low-voltage plasma andfurther changing a high-frequency pulse current to high-voltage toincrease emission of high-frequency high-voltage plasma and promotedissociation of ozone, efficiency of oxidizing soot contained in anexhaust gas can further be improved.

In the case when pulse currents having different voltage values are fed,the voltage value of each pulse current is not particularly limited. Forexample, in the case of treating an automobile exhaust gas, the pulsepower source is preferably able to feed a pulse current by switching thethird pulse current of 2 to 5 kV (low-voltage pulse current) and thefourth pulse current of 3 to 20 kV (high-voltage pulse current). Whenthe voltage value of the third pulse current (low-voltage pulse current)is below 2 kV, oxidation of NO is sometimes insufficient. When thevoltage value of the third pulse current (low-voltage pulse current) isabove 5 kV, hydrocarbon is sometimes oxidized completely. When thevoltage value of the fourth pulse current (high-voltage pulse current)is below 3 kV, oxidation of soot is sometimes insufficient. When thevoltage value of the fourth pulse current (high-voltage pulse current)is above 20 kV, the pulse power source is too large, which causes aproblem of mounting and sometimes causes a problem of electromagnetictrouble to the circumference.

Though a time interval for switching frequencies depends on the size ofthe pulse electrode, each frequency, each voltage value, etc., it ispreferably 0.01 to 500 seconds, and more preferably 0.01 to 10 seconds.A time interval for switching the above-mentioned frequencies may bedetermined by a pulse number in each condition. For example, in the caseof treating an automobile exhaust gas with feeding a pulse current byswitching between two kinds of frequencies, the time interval ispredetermined in such a manner that the pulse number of thelow-frequency pulse current is preferably 1 to 50000 times, morepreferably 3 to 50000 times, with respect to one pulse of thehigh-frequency pulse current having higher frequency. When the pulsenumber of the low-frequency pulse current with respect to one pulse ofthe high-frequency pulse current is below 1, production of aldehyde, orthe like effective in improving performance of a NO_(x) reductioncatalyst is sometimes insufficient. When the pulse number of thelow-frequency pulse current with respect to one pulse of thehigh-frequency pulse current is above 50000, time for collecting soot onthe ground electrode 5 becomes too long, and the soot sometimes cannotbe treated completely by high frequency plasma after switchingfrequency. Incidentally, the same applies in the case of voltage value.

Incidentally, as a pulse electrode, a ground electrode, and a powersource (pulse power source) for feeding a pulse current used for anexhaust gas treating method of the present embodiment, the pulseelectrode, ground electrode and pulse power source described in theembodiment of the first invention can suitable be employed.

Since components of an engine exhaust gas of an automobile greatlychanges in accordance with changes of driving conditions of an engine,various kinds of conditions for producing plasma adequate for treatingan exhaust gas, i.e., adequate values of voltage of a pulse current,frequency, time intervals for switching these, etc., are sometimesdifferent in accordance with this change. Therefore, in the presentembodiment, in the case of an exhaust gas discharged from an automobileengine, it is preferable to feed a pulse current by switching frequencyand/or voltage value for different values in stages at predeterminedtime intervals in correspondence with change in revolution and/or loadof the engine, and it is preferable that the predetermined time forswitching frequency and/or voltage value for different values is changedstep-by-step in correspondence with change in revolution and/or load ofthe engine. For example, it is preferable to previously determine valuesof two or more stages for frequency and/or voltage value and timeintervals for switching these in correspondence with change inrevolution and/or load of the engine so as to change frequency and/orvoltage value and time intervals for switching these according todriving conditions of the engine.

As the engine driving conditions, revolution and load are important, andby controlling various signals which relate to these, e.g., an outputsignal of a revolution meter, an output signal of an intake-airflowmeter, a control signal for a fuel injection amount, an acceleratoropening signal, an exhaust gas temperature signal, and an exhaust NO_(x)concentration signal, for example, frequency and/or voltage value andtime intervals for switching these can be controlled to one of thepreviously determined values in two or more stages. Signals for thecontrol are not limited to the signals described above, and it is, ofcourse possible to use another signal which relates to revolution andload.

In an exhaust gas of the present embodiment, an exhaust gas treatingapparatus 1 having a catalyst 10 may be disposed on downstream side of aportion where plasma is generated in an exhaust gas passage as shown inFIG. 2 to further treat an exhaust gas after passing the plasma. Asdescribed above, the exhaust gas treating apparatus 1 shown in FIG. 2 isprovided with a plasma producing means 3 disposed in an exhaust gaspassage and a catalyst 10 disposed on downstream side of the plasmaproducing means 3. A catalyst facilitating NO_(x) decompositionreaction, e.g., a NO_(x) reduction catalyst is used as the catalyst 10.By treating exhaust gas with such an exhaust gas treating apparatus 1,NO_(x) such as NO₂, which has difficulty in treating by plasma, can bedecomposed (reduced), and exhaust gas can be discharged outside in acleaner condition.

The present invention is hereinbelow described specifically withreferring to Examples. However, the present invention is by no meanslimited to these Examples.

Examples 1 to 7

Pulse electrodes and ground electrodes were formed by disposing atungsten conductive film by printing inside a ceramic body constitutedby alumina. Five pulse electrodes and five ground electrodes werealternately disposed oppositely inside a case body, and further a pulsepower source capable of switching frequencies and voltage values wasconnected to the pulse electrodes to obtain an exhaust gas treatingapparatus (Examples 1 to 7).

Each of the pulse electrodes and ground electrodes had dimensions of 90mm×50 mm×1 mm thick. Distance (mm) between the oppositely disposedelectrodes in the exhaust gas treating apparatus of each Example isshown in Table 1.

TABLE 1 Period A Distance Period B Gas concentration after treatmentbetween Voltage Number Voltage Number Presence Remaining electrodesvalue Frequency of pulse value Frequency of pulse HC NO NO₂ of Soot sootin (mm) (kV) (Hz) (time) (kV) (Hz) (time) (ppmC) (ppm) (ppm) aldehyde(mg/hr) apparatus Example 0.8 4 500 10 2 500 10 100 50 150 Present unde-Absent 1 tectable Example 0.8 4 500 3 2 500 10 150 50 150 Present unde-Absent 2 tectable Example 0.8 3 1000 10 2 500 10 100 50 150 Presentunde- Absent 3 tectable Example 0.8 3 1000 3 2 500 10 150 50 150 Presentunde- Absent 4 tectable Example 1.2 5 1000 3 3 500 10 160 50 150 Presentunde- Absent 5 tectable Example 1.2 3 1500 3 2 500 10 170 80 120 Presentunde- Absent 6 tectable Example 1.2 5 500 3 3 200 100000 190 60 140Present unde- Absent 7 tectable Comp. 0.8 4 500 — — — — unde- 40 150unde- unde- Absent Ex. 1 tectable tectable tectable Comp. 0.8 2 500 — —— — 150 50 150 Present 100 Present Ex. 2 Comp. 0.8 1 500 — — — — 200 16040 Present 1000  Present Ex. 3 Comp. 0.8 3 1000 — — — — unde- 40 150unde- unde- Absent Ex. 4 tectable tectable tectable Comp. 1.2 2 500 — —— — 190 100 100 Present 200 Present Ex. 5

As shown in Table 1, pulse currents having different frequencies andvoltage values was fed to an exhaust gas treating apparatus in each ofExamples 1 to 7 at two time intervals of Period A and Period B.Frequencies (Hz), voltage values (kV) in Period A and Period B and thenumbers of pulse (time) of pulse currents in each period are shown inTable 1.

An exhaust gas imitating an exhaust gas discharged from an engine wassent to the exhaust gas treating apparatus in each of Examples 1 to 7.The exhaust gas contained a mixed gas consisting of 10% by volume ofoxygen, 10% by volume of CO₂, 133 ppm of propylene, 67 ppm of propane,200 ppm of NO gas, and nitrogen as the rest and soot mixed in the gas at1000 mg/hr. There was measured concentration of hydrocarbon (HC),nitrogen monoxide (NO), and nitrogen dioxide (NO₂) contained in gasafter the treatment with an exhaust gas treating apparatus;presence/absence of aldehyde, and mass of soot after the treatment. Theresults are shown in Table 1. Incidentally, concentration of hydrocarbonis shown by mass in terms of carbon (ppmC).

The exhaust gas treated with an exhaust gas treating apparatus ofExamples 1 to 7 had reduced nitrogen monoxide (NO) and containedaldehyde, which has an effect in improving performance of a NO_(x)reduction catalyst. In addition, soot was also reduced to an amount lessthan the detectable limit of the detection apparatus.

Comparative Examples 1 to 5

There were produced an exhaust gas treating apparatuses (ComparativeExamples 1 to 5) having the same constitution as the exhaust gastreating apparatus of Example 1 except that the power sourceconstituting the exhaust gas treating apparatus can feed a pulse currenthaving only one kind of frequency and voltage value. Distance (mm),frequency (Hz) and voltage value (kV) of each space between adjacentelectrodes are shown in Table 1.

An exhaust gas was sent to each exhaust gas treating apparatus ofComparative Examples 1 to 5 in the same manner as in Example 1, andthere was measured concentration of hydrocarbon (HC), nitrogen monoxide(NO), and nitrogen dioxide (NO₂) contained in gas after the treatmentwith an exhaust gas treating apparatus; presence/absence of aldehyde,and mass of soot after the treatment. The results are shown in Table 1.Incidentally, concentration of hydrocarbon is shown by mass in terms ofcarbon (ppmC).

Though soot was not detected from the exhaust gas treated with anyexhaust gas treating apparatus of Comparative Examples 1 to 4, neitheraldehyde, which has an effect in improving performance of a NO_(x)reduction catalyst, nor hydrocarbon (HC) were detected. Though aldehydewas detected from the exhaust gas treated with an exhaust gas treatingapparatus of Comparative Examples 2 or 3, a large amount of soot wascontained.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, there can beprovided an exhaust gas treating apparatus and an exhaust gas treatingmethod capable of selectively treating substances to be treatedcontained in an exhaust gas.

1. An exhaust gas treating apparatus comprising: a case body functioningas a passage of exhaust gas containing substances to be treated, and aplasma producing means capable of producing plasma inside the case bodyfor treating the substances to be treated contained in the exhaust gas;wherein the plasma producing means has one or more each of a pulseelectrode and a ground electrode that are oppositely disposed in thecase body and has a pulse power source capable of feeding a pulsecurrent to the pulse electrode, and the pulse power source is capable ofautomatically switching frequency and/or voltage for different values atpredetermined time intervals so that plasma of a kind adequate for thesubstances to be treated contained in an exhaust gas is produced betweenthe pulse electrode and the ground electrode to selectively treat thesubstances to be treated in the exhaust gas.
 2. An exhaust gas treatingapparatus according to claim 1, wherein the predetermined time intervalsare 0.01 to 500 seconds.
 3. An exhaust gas treating apparatus accordingto claim 1, wherein the pulse power source is capable of feeding a pulsecurrent by switching from a first pulse current having a frequency of100 to 1000 Hz to a second pulse current having a frequency of 500 to2500 Hz, and vice versa.
 4. An exhaust gas treating apparatus accordingto claim 1, wherein the pulse power source is capable of feeding a pulsecurrent by switching from a third pulse current having a voltage valueof 2 to 5 kV to a fourth pulse current having a voltage value of 3 to 20kV, and vice versa.
 5. An exhaust gas treating apparatus according toclaim 1, wherein the pulse electrode and/or the ground electrodecomprise(s) a ceramic body functioning as a dielectric body and aconductive film disposed in the ceramic body.
 6. An exhaust gas treatingapparatus according to claim 1, which further comprises a catalyst on adownstream side of the plasma producing means in the passage of exhaustgas.
 7. The exhaust gas treating apparatus according to claim 1, whereinthe pulse power source is configured to switch the pulse current betweena low-voltage pulse current and a high-voltage pulse current, thelow-voltage pulse current and high-voltage pulse currents havingnon-zero voltages.
 8. An exhaust gas treating method for treatingsubstances to be treated contained in an exhaust gas by plasma producedin a passage of the exhaust gas containing the substances to be treated,the passage having one or more each of a pulse electrode and a groundelectrode that are oppositely disposed, the method comprising: switchingfrequency and/or voltage for different values at predetermined timeintervals to feed a pulse current that produces plasma between the pulseelectrode and the ground electrode, the plasma being of a kind adequatefor the substances to be treated contained in an exhaust gas; andselectively treating the substances to be treated in the exhaust gaswith the plasma.
 9. An exhaust gas treating method according to claim 8,wherein the predetermined time intervals are 0.01 to 500 seconds.
 10. Anexhaust gas treating method according to claim 8, wherein the pulsepower source is capable of feeding a pulse current by switching from afirst pulse current having a frequency of 100 to 1000 Hz to a secondpulse current having a frequency of 500 to 2500 Hz, and vice versa. 11.An exhaust gas treating method according to claim 8, wherein the pulsepower source is capable of feeding a pulse current by switching from athird pulse current having a voltage value of 2 to 5 kV to a fourthpulse current having a voltage value of 3 to 20 kV, and vice versa. 12.An exhaust gas treating method according to claim 8, wherein the exhaustgas is exhausted from an automobile engine, and the pulse current is fedby switching, step-by-step, frequency and/or voltage for differentvalues at predetermined time intervals in correspondence with change inrevolution and/or load of the engine.
 13. An exhaust gas treating methodaccording to claim 8, wherein the exhaust gas is exhausted from anautomobile engine, and the predetermined time intervals for switchingfrequency and/or voltage value for different values is changedstep-by-step in correspondence with change in revolution and/or load ofthe engine.
 14. An exhaust gas treating method according to claim 8,wherein a catalyst is further disposed on a downstream side of theportion where the plasma is produced in the passage of exhaust gas tofurther treat exhaust gas passed through the plasma by the catalyst. 15.The exhaust gas treating method according to claim 8, wherein theswitching step comprises switching the pulse current between alow-voltage pulse current and a high-voltage pulse current, thelow-voltage pulse current and high-voltage pulse currents havingnon-zero voltages.
 16. An exhaust gas treating apparatus, comprising: acase body functioning as a passage of exhaust gas containing substancesto be treated, and a plasma producer configured to produce plasma insidethe case body, wherein the plasma producer has one or more pulseelectrode and one or more ground electrode that are oppositely disposedin the case body and has a pulse power source capable of feeding a pulsecurrent to the one or more pulse electrode, and wherein the pulse powersource is configured to automatically switch frequency and/or voltagevalues at predetermined time intervals to produce plasma of a kindadequate to selectively treat the substances between the one or morepulse electrode and the one or more ground electrode.
 17. The exhaustgas treating apparatus according to claim 16, wherein the pulse powersource is configured to switch the pulse current between a low-voltagepulse current and a high-voltage pulse current, the low-voltage pulsecurrent and high-voltage pulse currents having non-zero voltages.